The study of arch dam-reservoir interaction is an outgrowth of a 4-year U.S.-China cooperative research project on 'Interaction Effects in the Seismic Response of Arch Dams.' Inconsistent comparisons were obtained in that project between measured and calculated dynamic reservoir pressures induced by shaking tests of arch dams; hence the study was planned to improve understanding of the dynamic interaction mechanism. Monticello Dam, an arch dam in California, was chosen as the test system, and the research involved comparison of hydrodynamic pressures measured during vibration tests with results predicted analytically. A major question for the study was the significance of compressibility of the reservoir water with regard to the interaction forces applied to the dam by the reservoir, so analyses were done both including and neglecting incompressibsity. Results of the study showed that hydrodynamic pressures measured at the face of the vibrating dam were in reasonable order-of-magnitude agreement with analytical results. Finally, the response of Monticello Dam to an earthquake appropriate to that location was calculated by two computer programs: ADAP-II which neglects compressibility and EACD-3D which includes it.